Mortgage Calculator with Extra Payments
Estimate how regular or one-off overpayments could change the time it takes to repay a mortgage and the interest paid, using the interest rate and term entered.
Estimated mortgage payoff time: 25 years
Add a regular or one-off overpayment to compare this illustration with the standard repayment schedule.
The mortgage is estimated to be repaid before the selected one-off overpayment period, so that payment is not included.
| Metric | Without overpayments | With overpayments |
|---|---|---|
| Scheduled payment | £3,595.49 per month | £3,595.49 per month |
| Estimated payoff time | 25 years | 25 years |
| Estimated interest paid | £478,646 | £478,646 |
| Interest difference | — | £0 less interest |
| Remaining balance after 5 years | £533,578 | £533,578 |
| Year | Scheduled payment | Extra payments | Principal repaid | Interest paid | Balance |
|---|---|---|---|---|---|
| 1 | £43,146 | — | £11,930 | £31,216 | £588,070 |
| 2 | £43,146 | — | £12,572 | £30,574 | £575,498 |
| 3 | £43,146 | — | £13,248 | £29,898 | £562,250 |
| 4 | £43,146 | — | £13,960 | £29,185 | £548,290 |
| 5 | £43,146 | — | £14,711 | £28,435 | £533,578 |
| 6 | £43,146 | — | £15,502 | £27,643 | £518,076 |
| 7 | £43,146 | — | £16,336 | £26,810 | £501,740 |
| 8 | £43,146 | — | £17,215 | £25,931 | £484,525 |
| 9 | £43,146 | — | £18,141 | £25,005 | £466,384 |
| 10 | £43,146 | — | £19,116 | £24,030 | £447,268 |
| 11 | £43,146 | — | £20,144 | £23,001 | £427,123 |
| 12 | £43,146 | — | £21,228 | £21,918 | £405,896 |
| 13 | £43,146 | — | £22,369 | £20,776 | £383,526 |
| 14 | £43,146 | — | £23,573 | £19,573 | £359,954 |
| 15 | £43,146 | — | £24,840 | £18,305 | £335,113 |
| 16 | £43,146 | — | £26,176 | £16,970 | £308,937 |
| 17 | £43,146 | — | £27,584 | £15,562 | £281,353 |
| 18 | £43,146 | — | £29,068 | £14,078 | £252,285 |
| 19 | £43,146 | — | £30,631 | £12,515 | £221,654 |
| 20 | £43,146 | — | £32,278 | £10,867 | £189,376 |
| 21 | £43,146 | — | £34,014 | £9,132 | £155,361 |
| 22 | £43,146 | — | £35,844 | £7,302 | £119,518 |
| 23 | £43,146 | — | £37,771 | £5,374 | £81,746 |
| 24 | £43,146 | — | £39,803 | £3,343 | £41,944 |
| 25 | £43,146 | — | £41,944 | £1,202 | £0 |
Estimated mortgage balance with and without overpayments
The orange line shows the balance in this overpayment illustration; the dark line shows the standard repayment schedule using the same rate and term.
Add an overpayment to compare the modeled balance with the standard repayment schedule.
| Metric | Without overpayments | With overpayments |
|---|---|---|
| Scheduled payment | £3,595.49 per month | £3,595.49 per month |
| Estimated payoff time | 25 years | 25 years |
| Estimated interest paid | £478,646 | £478,646 |
| Interest difference | — | £0 less interest |
| Remaining balance after 5 years | £533,578 | £533,578 |
Assumptions used
- Constant annual interest rate: 5.25%
- Mortgage term: 25 years
- Payment frequency: monthly
- Scheduled repayment remains unchanged
- Regular overpayment: None
- One-off overpayment: None
- No fees, early-repayment charges, overpayment allowances, rate changes, payment holidays or lender-specific rules included
How to use this calculator
Pick your currency, then enter the property price, your deposit, the annual interest rate and the mortgage term. Choose whether payments are made monthly or yearly. The calculator first builds a standard repayment schedule from those figures.
In the Overpayments (optional) section, add a regular extra amount for each period, a one-off lump sum, or both. When you enter a one-off amount, a field appears for the period it is made in — a number of months when payments are monthly, or years when they are yearly. Leave both overpayment fields blank to see the standard schedule on its own.
The result shows the estimated earlier repayment, the modeled payoff time and a comparison of interest paid with and without the overpayments. The Breakdown tab lists the annual schedule, the Chart tab plots both balance paths, and the Summary tab lists the assumptions used. This is a what-if illustration, not mortgage advice: it does not assess affordability, calculate early-repayment charges or tell you whether your lender allows an overpayment.
How mortgage overpayments are calculated
The calculator first creates a standard repayment schedule from the property price, deposit, interest rate, term and payment frequency. It then creates a second schedule using the same scheduled repayment and applies any extra payments to the balance after the scheduled payment for each period.
- P — loan amount, the property price minus the deposit
- r — the annual interest rate you enter, as a decimal (5.25% is 0.0525)
- m — payments per year: 12 for monthly or 1 for yearly
- i = r / m — interest rate per payment period
- n = T × m — original number of scheduled payments, for a term of T years
- M — the standard scheduled repayment, which does not change
- E — the regular overpayment each period (zero if none)
- L — the one-off overpayment (zero if none)
- k — the period the one-off overpayment is made in
- Bt — the balance at the end of period t, starting from B₀ = P
- If i = 0, M = P ÷ n
- interestₜ = Bₜ₋₁ × i
- actual overpaymentₜ = min(scheduled extraₜ, balance after the scheduled payment)
- Bₜ = balance after the scheduled payment − actual overpaymentₜ
For each period, interest is charged on the outstanding balance, the scheduled payment M is taken (reduced on the final period so the balance cannot go below zero), and then the extra payment — E, plus L in period k — is applied to the remaining balance. The extra payment is capped so the balance never becomes negative, and no further payments are charged once the balance reaches zero. The scheduled payment M is never recalculated in response to overpayments.
With a £600,000 loan at 5.25% over 25 years paid monthly, M is about £3,595.49. Adding £250 a month and a £10,000 lump sum after 12 months clears the modeled balance several years early and lowers the total interest charged in this constant-rate illustration. At a 0% rate, M is simply P ÷ n, every period’s interest is zero, and extra payments still shorten the modeled payoff time — but there is no interest difference to show.
Selecting a currency changes the labels and number formatting only. It does not convert amounts using exchange rates.
What this calculator does not do
It is an illustrative what-if calculation. It does not recommend overpaying, assess affordability, predict whether a lender will accept an overpayment, calculate early-repayment charges, apply an assumed overpayment allowance, recommend a mortgage term or product, forecast interest rates, or compare overpaying with investing or saving. Your mortgage offer and your lender’s rules determine whether overpayments are allowed, how they are applied and whether any charge applies.